In a flame atomic absorption spectrophotometer, a sample liquid which has been nebulized by a spray or in other manner and a combustion gas are mixed in a chamber. The mixed gas is burned while being flowed from a slit opening of a burner head to form a flame. In the flame, components (or elements) of the sample are atomized. When light is cast to the gaseous atoms, only the light of a predetermined wavelength in accordance with the species of the atoms is absorbed. By performing a spectroscopic analysis on this absorbed light, the components in the sample are identified and quantified.
As the combustion gas for making a flame, a mixed gas of a fuel gas, such as acetylene (C2H2), and a supporting gas, such as air or nitrous oxide (N2O), is generally used. Which of air or nitrous oxide is used as the supporting gas is determined in accordance with the element to be analyzed.
When a combustion gas normally burns, its burning rate and the flow rate of the gas flowing out from the burner head are in equilibrium, so that a flame is stably formed slightly above the upper surface of the burner head. However, if the flow rate of gas flowing out from the burner head is decreased for any reason, the balance between the burning rate and the gas flow rate is lost, so that the flame is drawn into the burner head. This phenomenon is called a “flashback,” which is a small-scale explosion produced when the combustion zone of the flame enters into the burner head or into the chamber filled with the combustion gas.
The burning rate is determined by the mixture ratio of the fuel gas and the supporting gas and the temperature around the flame. The burning rate becomes largest (or maximum) at the stoichiometric ratio (at which the reaction gases are completely burned). When a flashback occurs, the magnitude of explosion (or energy) of the flashback becomes larger as the mixture ratio of the gases at that point is closer to the stoichiometric ratio. Major causes of flashback include, for example, a decrease in the supply pressure of the fuel gas or supporting gas (or decrease in the pressure of the tank) and an unexpected failure of a supply pipe of the gas.
Since a flame atomic absorption spectrophotometer uses combustive gases as previously described, a variety of measures for safety are conventionally taken. For example, in the apparatus described in Patent Document 1, the gas supply pressure is always monitored, and in the case where the pressure of the gas decreases while a flame is burning, the supply of the gas is automatically halted in order to safely extinguish the flame. As described earlier, a decrease in the pressure of gas supply is one of the causes of the flashback. Therefore, detecting a decrease in the pressure of gas supply and extinguishing the flame can prevent a flashback.
In the apparatuses described in Patent Documents 2 and 3, and other documents, in order to prevent the fuel gas from leaking after the flame goes out, the intensity of light of the flame is always monitored while burning. When the intensity of light falls below the value of normal burning, the flame is automatically extinguished and the supply of the fuel gas is halted. Such a safety mechanism is employed in actual apparatuses to ensure their safety, as in Atomic Absorption Spectrophotometer AA6300, the details of which are disclosed on Shimadzu Corporation's website.
In practice, even with such safety measures as previously described, it is difficult to completely prevent a flashback, and a flashback could still occur with a very low probability. Since, in a conventional flame atomic absorption spectrophotometer, the flame goes out when a flashback occurs, the safety mechanism as previously described halts the supply of the fuel gas and the supporting gas. Thus, the gases are prevented from leaking If, after the gas supply to the burner is halted and the flame is extinguished in this manner, the operator performs an ignition operation again by pressing the ignition button, the fuel gas and supporting gas is supplied to the burner again and the flame is created in accordance with a predetermined ignition sequence. That is, in a conventional flame atomic absorption spectrophotometer, even after a flashback occurs and the flame is once extinguished, the apparatus can be resumed in use.
However, in the event of a flashback, albeit depending on the scale of the explosion, the burner head may fall apart from the chamber or the burner head may be damaged by the energy of the explosion. In some cases, the chamber itself or the gas supply pipes might be broken. If, on the other hand, the burner head, chamber, gas supply pipe, or other unit is broken so slightly as to be apparently unrecognizable, the operator might resume using the apparatus without knowing the damage in the apparatus, which might lead to a serious secondary damage.
If the operator of the apparatus is an engineer or an expert having a compiled knowledge of analysis, he/she often recognizes the problems that may occur in case of a flashback. Hence, the operator does not restart the apparatus immediately after the flashback, but can smoothly take an appropriate action such as reporting the event to a maintenance staff of the apparatus. However, in recent years, an increasing number of unskilled operators and persons having less knowledge have come to operate a flame atomic absorption spectrophotometer, who might restart the apparatus without a careful consideration. Under such circumstances, measures for higher safety than before are required in flame atomic absorption spectrophotometers.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-183376    Patent Document 2: Japanese Unexamined Patent Application Publication No. H07-294424    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-69763